Liquid crystals are widely used for electronic displays. In these display systems, a liquid crystal cell is typically situated between a polarizer and an analyzer. An incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the analyzer. By employing this principle, the transmission of light from an external source including ambient light, can be controlled. The energy required to achieve this control is generally much less than required for the luminescent materials used in other display types such as cathode ray tubes (CRT). Accordingly, liquid crystal technology is used for a number of electronic imaging devices, including but not limited to digital watches, calculators, portable computers, and electronic games for which light-weight, low-power consumption and long-operating life are important features.
Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display (LCD) is the propensity for light to “leak” through liquid crystal elements or cells, which are in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the direction from which the display screen is viewed (“viewing angle”). Typically the optimum contrast is observed only within a narrow viewing angle range centered about the normal incidence to the display and falls off rapidly as the viewing direction deviates from the display normal. In color displays, the leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction.
LCDs are quickly replacing CRTs as monitors for desktop computers and other office or household appliances. It is also expected that the number of LCD television monitors with a larger screen size will sharply increase in the near future. However, unless problems of viewing angle dependence such as hue shift, degradation in contrast, and an inversion of brightness are solved, the LCD's application as a replacement of the traditional CRT will be limited.
A Vertically-Aligned liquid crystal display (VA-LCD) offers an extremely high contrast ratio for normal incident light. FIG. 10A and FIG. 10B are the schematics of a VA liquid crystal cell in OFF 1001 and ON 1003 states. In its OFF state, the liquid crystal optic axis 1005 is almost perpendicular to the substrate 1007, FIG. 10A. With an applied voltage, the optic axis 1005 is tilted away from the cell normal, FIG. 10B. In the OFF state, light in the normal direction 1009 does not see the birefringence of the liquid crystal layer, yielding a dark state that is close to that of orthogonally crossed polarizers. However, obliquely propagated light 1011 picks up retardation from the liquid crystal layer, producing light leakage. This results in a poor contrast ratio in some viewing angle range.
A bend aligned nematic liquid crystal display, also referred as an Optically Compensated Bend Liquid Crystal Display (OCB-LCD) uses a nematic liquid crystal cell based on the symmetric bend state. In its actual operation, the brightness of the display using the bend aligned nematic liquid crystal cell is controlled by an applied voltage or field that leads to a different degree in the bend orientation within the cell as shown in FIG. 11A (OFF) 1101 and FIG. 11B (ON) 1103. In both states, the liquid crystal optic axis 1105 takes on a symmetric bend state around the cell middle plane 1107. In the ON state, the optic axis becomes substantially perpendicular to the cell plane except near the cell substrates 1109. OCB mode offers faster response speed that is suitable to the liquid crystal display television (LCD-TV) application. It also has advantages in viewing angle characteristic (VAC) over conventional displays, such as Twisted Nematic liquid crystal display (TN-LCD)
The above-mentioned two modes, due to their superiority over the conventional TN-LCD, are expected to dominate the high-end applications such as LCD-TV. However, practical applications of both OCB-LCDs and VA-LCDs require optical compensating means to optimize the VAC. In both modes, due to the birefringence of liquid crystal and the crossed polarizers, the VAC suffers deterioration in contrast when the displays are viewed from oblique angles. The use of biaxial films has been suggested to compensate the OCB (U.S. Pat No. 6,108,058) and VA (JP1999-95208) LCDs. In both modes, liquid crystals align sufficiently perpendicular to the plane of the cell in ON (OCB) or OFF (VA) states. This state gives positive out-of-plane retardation, Rth, thus the compensation films have to have sufficiently large negative Rth for satisfactory optical compensation. The need for a biaxial film with a large Rth is also common for Super Twisted Nematic Liquid Crystal Display (STN-LCD).
Another promising type of LCD is the in-plane switching mode LCD. In the VA-LCD and OCB-LCD devices discussed above the electrodes are disposed on opposite sides of the LC layer, that is, on the opposing substrates. In contrast, in an in-plane switching mode LCD, electrodes are disposed on a same side of the LC layer, that is, on a same substrate. However, in order to improve oblique angle contrast, an in-plane switching device needs an optical compensator with a sufficiently large positive out-of-plane retardation Rth. In particular, multilayer compensators where the (Rth) of the multilayer compensator is more positive than +20 nm would be useful in compensating in-plane switching (IPS) mode LCD's.
Several methods of manufacturing biaxial films with a sufficient negative value of Rth suitable for compensating LCD modes such as OCB, VA and STN have been suggested.
Sasaki et al. proposes (US2003/0086033) the use of cholesteric liquid crystal disposed on a positively birefringent thermoplastic substrate. The pitch of the cholesteric liquid crystal (CHLC) is shorter than the wavelength of the visible light, thus properly aligned CHLC exhibits form birefringence giving negative Rth. Rin is controlled by adjusting the stretching amount of the thermoplastic substrate. The method enables one to adjust Rth and Rin separately. However, the use of short pitch CHLC not only makes the manufacturing cost high but also complicates the processing due to the alignment procedure.
Wada et al. (EP09544013A1) disclose an optical compensator including an optically compensating film that is laminated to an optically isotropic film using, for example, a urethane adhesive. Wada teach that only certain polymers are suitable for their optically compensating film, and in particular, teach that certain common, inexpensive materials such as polycarbonate and polystyrene should not be used.
US 2001/0026338 discloses the use of a retardation-increasing agent in combination with triacetylcellulose (TAC). The retardation-increasing agent is chosen from aromatic compounds having at least two benzene rings. By stretching the agent-doped-TAC, one can generate both Rth and in-plane retardation, Rin. However, one problem with this method is the amount of the doping agent required. To generate the desired effects of increasing Rth and Rin, the necessary amount of agent can be high enough to cause unwanted coloration, or movement (diffusion) of the agent into other layers in the LCD with a resulting loss of Rth and Rin and undesired chemistry in these adjacent layers. Also, with this method it is difficult to control the values of Rth and Rin independently.
JP2002-210766 discloses the use of propionyl or butyryl substituted TAC. They show higher birefringence than ordinary TAC. Thus, by biaxially stretching the substituted TAC film, one can generate Rin and Rth. The method does not require any additional coating or layer, but it suffers from a difficulty of independent control of Rin and Rth.
Moreover, the TAC based structures described above suffer from other disadvantages. For one thing, the TAC film must have a thickness greater than at least 40 μm, as it is very difficult to reliably fabricate films that are much thinner than that by the casting methods typically employed, due to the fragility of wet film during the peeling and drying process. In practice, the TAC film will typically have a thickness of at least 80 μm to provide adequate structural stability. Meanwhile however, as the demand increases for thinner and thinner displays, it becomes necessary to produce thinner optical compensation films.
Another drawback to the casting method of forming TAC base structures is the inability to accurately apply multiple layers. As noted in U.S. Pat. No. 5,256,357 to Hayward, conventional multi-slot casting dies create unacceptably non-uniform films. In particular, line and streak non-uniformity is greater than 5% with prior art devices. Acceptable two layer films may be prepared by employing special die lip designs as taught in U.S. Pat. No. 5,256,357 to Hayward, but the die designs are complex and may be impractical for applying more than two layers simultaneously.
Yet another drawback to the casting method is the restrictions on the viscosity of the dope. In casting practice, the viscosity of dope is on the order of 50,000 cp. For example, U.S. Pat. No. 5,256,357 to Hayward describes practical casting examples using dopes with a viscosity of 100,000 cp. In general, cast films prepared with lower viscosity dopes are known to produce non-uniform films as noted for example in U.S. Pat. No. 5,695,694 to Iwata. In U.S. Pat. No. 5,695,694 to Iwata, the lowest viscosity dopes used to prepare casting samples are approximately 10,000 cp. At these high viscosity values, however, casting dopes are difficult to filter and degas. While fibers and larger debris may be removed, softer materials such as polymer slugs are more difficult to filter at the high pressures found in dope delivery systems. Particulate and bubble artifacts create conspicuous inclusion defects as well as streaks and may create substantial waste.
In addition, the casting method can be relatively inflexible with respect to product changes. Because casting requires high viscosity dopes, changing product formulations requires extensive down time for cleaning delivery systems to eliminate the possibility of contamination. Particularly problematic are formulation changes involving incompatible polymers and solvents. In fact, formulation changes are so time consuming and expensive with the casting method that most production machines are dedicated exclusively to producing only one film type.
Cast films also may exhibit undesirable cockle or wrinkles. Thinner films are especially vulnerable to dimensional artifacts either during the peeling and drying steps of the casting process or during subsequent handling of the film. In particular, the preparation of polarizer plates from resin films requires a lamination process involving application of adhesives, pressure, and high temperatures. Very thin films are difficult to handle during this lamination process without wrinkling. In addition, many cast films may naturally become distorted over time due to the effects of moisture. For optical films, good dimensional stability is necessary during storage as well as during subsequent fabrication of polarizer plates.
Thus, it would be desirable to provide an optical compensator with independently controlled Rth and Rin that can be readily and inexpensively manufactured. It would also be desirable to provide an optical compensator capable of a greater range of in-plane retardance, Rin. It would further be desirable to provide a thinner optical compensator, and a method of producing a thinner optical compensator. It would be still further desirable to provide a method of producing an optical compensator that mitigates one or more of the disadvantages of the traditional casting processes as described above.